1. Field of the Invention
The present invention relates to electrodes for solid oxide electrochemical cells and more specifically to a method of fabricating electrodes on solid oxide electrochemical cells by sintering. In this case, electrochemical cells include fuel cells, electrolyzers and sensors that operate on the basis of electromotive force measurement and/or current measurement and which comprise a solid oxide electrolyte and attached electrodes. Solid oxide fuel cells are one typical field of application of this invention. Although this invention was developed specifically for the fabrication of electrodes on fuel cells, it may also be used to fabricate electrodes on a variety of other electrochemical devices.
2. Description of the Prior Art
Solid oxide fuel cells are high temperature electrochemical devices fabricated primarily from oxide ceramics. Typically, they contain an oxygen ion conducting solid electrolyte, such as stabilized zirconia. The electrolyte is usually a thin dense film which separates two porous electrodes--an anode and a cathode. The cathode, which is maintained in an oxidizing atmosphere, is usually an oxide doped for high electrical conductivity, such as strontium doped lanthanum manganite. The anode, on the other hand, is maintained in a reducing atmosphere and is usually a cermet such as nickel-zirconia. Finally, an interconnection is usually employed which is a dense, electronically conducting oxide material which is stable in both reducing and oxidizing environments, such as doped lanthanum chromite. The interconnection is deposited on a cell as a thin gas tight layer in such a manner that it permits the anodes and cathodes of adjacent cells to be electrically connected in series. The gas-tightness of the interconnection, in combination with that of the electrolyte, insures that the entire cell is gas-tight, preventing mixing of the anode and cathode atmospheres.
Solid oxide cells can be operated in either an electrolysis mode or in a fuel cell mode. In an electrolysis mode, DC electrical power and steam or carbon dioxide or mixtures thereof are supplied to the cell which then decomposes the gas to form hydrogen or carbon monoxide or mixtures thereof, as well as oxygen. In the fuel cell mode, the cell operates by electrochemically oxidizing a gaseous fuel such as hydrogen, carbon monoxide, methane or other fuels to produce electricity and heat.
The use of nickel-zirconia cermet anodes for solid oxide electrolyte fuel cells is well known in the art, and taught, for example, by A. O. Isenberg in U.S. Pat. No. 4,490,444. The anode must be compatible in chemical, electrical, and physical-mechanical characteristics such as thermal expansion, to the solid oxide electrolyte to which it is attached. A. O. Isenberg, in U.S. Pat. No. 4,597,170 solved bonding and thermal expansion problems between the anode and solid oxide electrolyte, by use of a skeletal embedding growth, of for example, primarily ionically conducting zirconia doped with minor amounts of yttria. The skeletal growth extends from the electrolyte/anode interface into a porous nickel layer, with the composite structure comprising the porous cermet anode.
Anchoring of the porous nickel anode to the solid oxide electrolyte was accomplished by a modified electrochemical vapor deposition (EVD) process. While this process provided well bonded anodes, having good mechanical strength and thermal expansion compatibility, gas diffusion overvoltages were observed during operation, lowering overall cell performance.
In order to reduce gas diffusion overvoltages, A. O. Isenberg et al., in U.S. Pat. No. 4,582,766, taught oxidizing the nickel in the cermet electrode to form a metal oxide layer between the metal, and the electrolyte, the embedding skeletal member. Subsequent reduction of the metal oxide layer forms a porous metal layer between the metal, and the electrolyte and skeletal member allowing greater electrochemical activity. The EVD process, while producing acceptable quality electrodes is labor-intensive. What is needed is a low cost process for the fabrication of active anode structures in order to eliminate the need for electrochemical vapor deposition.